Abrasive tools are generally classed according to the way in which the ceramic grits of which they are composed are formed: free abrasives (use in spraying or in suspension, without a support), coated abrasives (with a support of the cloth or paper type, in which the grits are located on several layers) and bonded abrasives (in form of circular grinding wheels, of sticks, etc.). In the latter case, the abrasive grits are pressed with an organic or vitreous binder (for example, a binder consisting of oxides, essentially silicates). These grits must themselves have good mechanical abrasion properties (especially toughness) and give rise to good mechanical cohesion with the binder (interfacial strength). At the present time, there are commercially available various families of abrasive grits making it possible to cover a wide range of applications and of performance: oxide grits synthesized by fusion (called fused grits here) in particular offer an excellent quality/manufacturing cost compromise.
Within the fused grit range, materials based on alumina and zirconia are known from U.S. Pat. No. 3,181,939. These grits are generally composed, by weight, of 10 to 60% of zirconia and 0 to 10% of an additive, the balance being alumina. In practice, the zirconia content of the commercial products lies either around 25% or around 35 to 50%, that is to say around the composition of the alumina-zirconia eutectic located at about 42% zirconia, as described in U.S. Pat. No. 3,891,408. The latter reference indicates that the products around the eutectic offer better abrasive performance than alumina grits, especially if they have been very rapidly solidified so that most of the structure is composed of eutectic colonies and that, in the eutectic colonies, the interlemellar or interfiber spaces are less than 4000 Å, the eutectic colonies being oriented perpendicular to the solidification front. This type of unique structure provides, for abrasive applications, an excellent balance between mechanical strength, required for maximum use of the grit, and microfracturing during use, necessary for good regeneration of the cutting surfaces. Moreover, it is known that it is preferable to use products in which the zirconia is present in its tetragonal (or even cubic) allotropic form and not in its monoclinic form. Stabilizers, such as yttrium oxide added in an amount up to 2% according to U.S. Pat. No. 4,457,767 for titanium oxide added in an amount up to 10% according to DE-C-4 306 966, are also known to improve the abrasive power of the alumina-zirconia grits. Magnesium oxide has also been mentioned as a possible additive, but above a content of a few percentage points, its presence leads to the formation of a spinel with alumina until disappearance of the corundum, and hence a lower mechanical performance.
Alumina-zirconia grits are also the grits of choice for refractory applications, particularly for the manufacturer of nozzles or slide gate valve plates employed in the continuous casting of steel. The grits are incorporated in matrices often containing carbon (“black” products) but also in oxide matrices (“white” products and frits), especially alumina matrices. For refractory applications, resistance to the heat treatment is essential, whether during the forming of the components, or in service. Consequently, apart from the requirement of having low thermal expansion coefficients (so as to minimize the stresses on the matrix), the grit must have, after calcining, mechanical properties sufficient for itself not to result in the destruction of the component. These requirements very advantageously position the alumina-zirconia grits containing, as in the case of abrasive applications, around 25 or 42% zirconia. In particular, compositions containing 42% zirconia have good mechanical strength, a low thermal expansion coefficient and good corrosion resistance. The latter is improved in products based on dense and crack-free grits. This is because pores and cracks in this application form regions where molten liquids preferably penetrate and hence preferred sites of corrosion. Of course, the good mechanical strength of the material also requires a low content of defects, such as pores or cracks. For this purpose, in order to avoid volume changes and the associated cracks, which are induced by the transformation at temperature of monoclinic zirconia, the zirconia may be stabilized by the addition of dopants, such as titanium oxide or yttrium oxide, possibly combined with quenching of the product, which favors stabilization of the zirconia in the tetragonal, or even cubic, form.
From the process standpoint, the material is produced by fusing the raw materials of variable purity under reducing conditions, especially by the addition of a source of carbon (petroleum coke, pitch or coal) in the furnace. It is preferred in general to cool the material rapidly in order to favor the formation of fine and oriented structures using equipment, such as that described in U.S. Pat. No. 3,993,119, for casting between thin metal plates. Finally, the material produced is milled, for example in roll mills, and then screened and classified into series of grit size distributions meeting precise specifications (for example FEPA).
Producing the material under reducing conditions helps to reduce the impurity content of the cast product and makes it possible to obtain a dense material having good mechanical properties. Further beneficial effects may be mentioned, especially with regard to stabilization of tetragonal zirconia (see DE-C-4 306 966). In any event, the grades preferred for applications (organic wheels, “black” refractories or coated abrasives) in the prior art are always reduced and consequently contain carbon, sub-oxides and/or metallic or carbide species. U.S. Pat. No. 3,891,408 clearly indicates that the least reduced products [containing less than 0.5% (5000 ppm) carbon] exhibit lower abrasion performance. U.S. Pat. No. 5,143,522 mentions products containing from 300 to 5000 ppm carbon (examples according to the invention) and 100 or 200 ppm (comparative examples of lower performance). These high contents indicate the reduced state of the grits: they guarantee good performance, whether in coated abrasives or in wheels with organic binder.
However, the use of these reduced alumina-zirconia grits for producing wheels with a vitreous mineral binder has not been successful because of an incompatibility between these grits and the vitreous binders typically used in bonded abrasives and, to the Applicant's knowledge, no article of this kind exists on the market. The wheels with a vitreous mineral binder commercially available at the present time are produced from fused or sintered corundum grits.
From the studies carried out by the Applicant, the incompatibility between the grits and the vitreous binder is due to the highly reduced state of these grits. This is because we have found that a ceramic grit which is reduced too much reacts during firing of the vitrified wheels, releasing bubbles into the binder, which significantly reduces the mechanical properties of the wheel. A highly reduced grit even leads to a large volume expansion of the wheel, which is immediately perceptible to the eye. Without wishing to be tied to any particular theory, we believe that the contact in an oxidizing atmosphere at high temperature (above 900° C. for several hours) of a sub-oxidized product based on alumina and zirconia with a number of oxides of lower stability (such as silicate binders) naturally gives rise to redox reactions, these being accompanied by the evolution of gases and/or large expansions which weaken the grit and its interface with the binder of the wheel. Similarly, for refractory applications, the resistance to grit reoxidation is a particularly important aspect, both in production (in the case of “white” refractories) and during use (oxidation by slag in the case of all steelmaking refractories). Reduced grits are not suitable for uses as white products, these being, in fact, the seat of multiple fracturing and volume expansions unacceptable during use of the components. More generally, for all steelmaking refractories, reduced grits are inconducive to resistance to oxidation by slag.
To overcome these problems, we have considered producing fused alumina-zirconia grits which are less reduced.
A first approach consisted in subjecting “conventional” (reduced) materials to a heat treatment in an oxidizing atmosphere once they have solidified. However, we have observed that this results in a drop in the performance of said grits to a point at which there is no longer any benefit of the alumina-zirconia material; this is because the mechanical tests show (see the examples) that the conventional grits thus treated have a lower performance than that of the fused corundum control products. This confirms the discrete tests detailed in U.S. Pat. No. 3,891,408 or DE-C-4306966 which indeed indicate a drop in performance due to a heat treatment.
A second approach has consisted in oxidizing the melt pool before casting, using known techniques such as that of controlling the fusion energy in the case of furnaces open to the air (the product obtained will be more oxidized the greater the amount of energy supplied during fusion), of adjusting the length of the arcs, of injecting oxidizing gases, etc. However, we have found that such production conditions generally result in porous materials, which lowers the mechanical performance of these materials.
As a variant, the molten liquid may be oxidized upon solidification, especially by air dispersion. However, reoxidation of a reduced liquid produces blisters (porosity) at the surface in contact with the ambient air during solidification, whether on granulates obtained by air dispersion or at the upper surface of the casting molds.
Finally, it may be noted that, in order to alleviate the problem of porosity, another possible solution would be to add a few percent of silica to the composition. Thus, it appears that a silicate phase helps to reduce porosity. However, the presence of silica is deleterious to the mechanical properties of the grits. U.S. Pat. No. 3,891,408 and U.S. Pat. No. 5,143,522 indicate, moreover, that appreciable amounts of SiO2 (1% maximum in the case of these two patents) or of Na2O (0.1% maximum) should be avoided.
The abovementioned approaches are the simplest to implement but, unfortunately, the products which stem therefrom do not have the required characteristics.